![]() ![]() Average values for the Chalk formations range up to about 30 API units, whereas those for the mudstone formations generally exceed 35 API units. The differentiation of mudstone-dominated and limestone-dominated facies is achieved on the basis of gamma-values. The Shetland Group now includes all formations dominated by mudstone facies and the Chalk Group all formations dominated by chalky limestone facies. In the UK Sector Northern North Sea Johnson & Lott (1993) followed Deegan & Scull (1977) in recognizing both the Chalk and Shetland groups, albeit with modified definitions (Table 4.2). The nomenclature used here is derived from columns 3, 4, 5. Table 1: Historical development of the Upper Cretaceous lithostratigraphical nomenclature of the offshore UK Sector (from Hopson 2005). The Shetland Group has been extended in the Norwegian Sea such that it has a similar stratigraphic duration and boundaries to the original definition in the North Sea by Deegan & Scull (1977), (Table 1). The original definitions in the Norwegian Sector of the North Sea are in Dalland et al. They did not define formations within the Shetland Group, but recognized six informal subdivisions and designated these 'Formations' A to F (Table 1). The term Shetland Group was introduced by Deegan & Scull (1977) for a unit of mudstone with relatively minor interbedded argillaceous limestones lying between the Cromer Knoll and Montrose groups in the Northern North Sea. Newsletter on Stratigraphy, vol 49/1 pp71-280, 2016 Introduction Waters (editors), Mike Charnock, Dirk Munsterman, Michelle Hollerbach, Harald Brunstad, Øyvind Hammer & Luis Vergara (contributors). Updated to follow: Stratigraphic Guide to the Cromer Knoll, Shetland and Chalk Groups of the North Sea and Norwegian Sea. ![]()
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